Most of them have colocation with the exchange, so if you are talking about prop firms like Jane Street, SIG, DRW, Optiver etc, it's prob. microseconds. The data and execution is so fast it really doesn't matter unless you are comparing to other firms. If you are talking about firms with prop trading like Goldman Sachs, they most likely have the infrastructure to be just as fast, if not faster. Not only that, nearly all the prop firms specialize in low-latency, and they optimize/tweak/rewrite parts of the TCP/UDP/IP protocol to ensure the lowest latency. So speed is actually dependent upon what safety mechanisms you want in your connections (example being do you need reliability?) Special hardware custom built for these firms also allow this huge speed; here's an example. http://www.cisco.com/web/strategy/financial/algo_speed.html#~one
nice info. i'm in chicago, and I wonder then what kind of connection do i need to access my colo from my future office. would the building management provide some sort of arrangement to link up or get the comcast guys